Relatively little is known about the impact of global warming on the tropical cyclone (TC) outflow, despite its large contribution to TC intensity. In this study, based on the International Best Track Archive for Climate Stewardship (IBTrACS) dataset and ERA5 reanalysis data, we show that the TC outflow height has risen significantly (48.20 ± 22.18 m decades−1) in the past decades (1959–2021) over the western North Pacific, and the rising trend tends to be sharper for stronger TCs (the uptrend of severe typhoon is 61.09 ± 40.92 m decades−1). This rising trend of the outflow height explains the contradiction between the decrease trend of the TC outflow temperature and the increase trend of the atmospheric troposphere temperature. Moreover, possible contribution of the TC outflow height uptrend to TC intensity has also been investigated. The results show that the rise of outflow height leads to the decrease of outflow temperature, and thus an increased difference between underlying sea surface temperature (SST) and TC outflow temperature, which eventually favors the increase of TC intensity.
尽管热带气旋(TC)外流层对TC强度有很大影响，但全球变暖背景下TC外流层的变化却知之甚少。本研究基于IBTrACS数据集和ERA5再分析数据，发现过去60余年(1959–2021年)北太平洋西部的TC外流层高度显著上升(48.20 ± 22.18 m decades−1)，且较强TC外流层高度的上升趋势更明显(强台风外流层高度的上升趋势为61.09 ± 40.92 m decades−1)。这种外流层高度的上升趋势解释了TC外流层温度下降趋势与全球变暖影响下大气对流层温度上升之间的矛盾。此外，本文还研究了TC外流层高度上升趋势对TC强度变化的可能贡献。结果表明，外流层高度的上升会导致外流层温度的下降，从而使海面温度与TC外流层温度之间的差值增大，最终有利于TC强度的增加。
The western North Pacific (WNP) is the most active region for tropical cyclones (TCs), accounting for about 30% of the global TCs (Emanuel, 2005; Emanuel et al., 2006). TCs in the WNP can have drastic economic consequences on southeastern China, the Philippine Islands, Japan, and the maritime routes between these areas (Pielke et al., 2008; Peduzzi et al., 2012). Previous studies showed that global warming may play an important role in changing TC activities. In the past decades, not only surface sea temperature (SST) increases, but also atmospheric circulation [e.g., western Pacific subtropical high (WPSH)] changes (Knutson et al., 2010; Sun et al., 2017a; He and Zhou, 2020). Unlike the sea–air exchange between surface and mid levels (e.g., SST and vertical wind speed) of TCs, a topic that has been discussed frequently in the literature for nearly 60 yr (Palmén and Riehl, 1957), the outflow layers of TC are not as active an area of study, especially for TCs in the WNP.
Previous studies have found that TC activities are affected by atmospheric thermodynamics, especially the upper tropospheric temperature (e.g., Sun et al., 2014). Generally, the upper tropospheric temperature rises and falls consistently with the underlying SST in the tropics (Vecchi and Soden, 2007; Vecchi et al., 2008). Both the upper tropospheric temperature and the SST increased with the influence of global warming (Chan and Wu, 2005; Wu et al., 2015). However, Emanuel et al. (2013) found that the TC outflow temperature has decreased in the past decades, which favors the development of TC in North Atlantic. Besides, Ge et al. (2018) indicated that the decrease in TC outflow temperature can contribute to TC rapid intensification. In the past decades, there seems to be a contradiction between the decreasing trend of the TC outflow temperature and the increasing trend of the tropospheric air temperature.
In this paper, we use the International Best Track Archive for Climate Stewardship (IBTrACS) dataset and ERA5 reanalysis data over 1959–2021 to conduct an analysis on the TC-related changes in the upper troposphere (especially the TC outflow layer) in the past decades. The results show that the troposphere temperature increases while the TC outflow temperature decreases, which is consistent with previous studies. Note that our study found the outflow height of TC rising significantly, and this change is more obvious for strong TCs. We calculate the contribution of TC outflow height and troposphere temperature changes to TC outflow temperature change, respectively, which shows that the influence of rising outflow height is much greater than the warming of troposphere temperature and may partly explain the opposite trends of TC outflow temperature and troposphere temperature. Moreover, we also investigate the possible contribution of TC outflow height and temperature to TC potential intensity.
The rest of this paper is organized as follows. In Section 2, we describe data sources and calculation methods. The experiments and analysis results are presented in Section 3. Finally, conclusions and discussion are given in Section 4.
2.
Data and methods
2.1
IBTrACS dataset
The TC data used in this study are from all numbered TCs and all unnumbered influential TCs over the WNP recorded in the IBTrACS dataset. IBTrACS is a global archive of TC best track data, which uses remote sensing data such as satellite imagery, microwave, and infrared imagery, scatterometry, and other data sources to determine the best track of TCs (Kossin et al., 2014). The IBTrACS dataset has a temporal resolution of 3 h. It records the longitudes and latitudes, the central minimum pressure, and the 2-minute average near-center maxi-mum wind speed of the TCs from 1959 to 2021. This paper mainly uses the IBTrACS data to obtain information such as TC position and intensity.
2.2
ERA5 reanalysis data
ERA5 is a global atmospheric reanalysis dataset produced by the ECMWF. ERA5 combines observations from various sources, including satellite data, surface observations, and weather balloon data, with a numerical weather prediction model to provide a detailed picture of the state of the atmosphere. Data are available on a horizontal resolution of 0.25° × 0.25° and a vertical resolution of 25 hPa at 3-h intervals from 1959 to 2021 over the WNP. The ERA5 dataset contains a wide range of atmospheric variables, including temperature, pressure, humidity, wind speed and direction, and precipitation, among others.
2.3
Revision of TC positions
For a specific TC, we first determine its latitude, longitude, and whole life cycle period from the IBTrACS dataset and compare them with the TC information in the ERA5 reanalysis data. In the comparison, we found that some TCs in the IBTrACS dataset are missing in the ERA5 data, and some TCs have shifts in their center locations between the two datasets. We removed the TCs that did not appear simultaneously in both datasets and revised the TCs with shifted center positions based on the ERA5 reanalysis data (Kim et al., 2015; Sun et al., 2017b). We investigate the TC outflow layer and TC activities using the revised TC position and time information. Figure 1 shows the revised TC tracks in this study.
Similar to Sun et al. (2017b) and Kim et al. (2015), the TC identification criteria in the ERA5 reanalysis data are as follows: (1) the maximum relative vorticity at 850 hPa exceeds 4.0 × 10−5 s−1; (2) the maximum wind speed at 10 m within a radius of 800 km from the TC center is greater than 10.8 m s−1; (3) the lifetime maximum wind speed at 10 m is greater than 17.2 m s−1; (4) TC-center temperature at 300 hPa is at least 1°C higher than the area-averaged temperature within a radius of 200–600 km from the TC center; (5) TC lifetime must be at least 48 h, and (6) the TC genesis is located south of 30°N over the ocean. Note that the grid point with the mini-mum sea-level pressure within a radius of 500 km from the position of the maximum relative vorticity is defined as the center of the TC.
Fig
1.
Revised TC tracks (blue lines) based on the IBTrACS dataset and adjusted to the ERA5 reanalysis data (total TC number: 1892).
2.4
TC outflow height and temperature
Previous studies showed that the TC outflow height is at 100–300 hPa (Merrill and Velden, 1996; Komaromi and Doyle, 2017). We calculated the TC radial velocity within 100–300 hPa (total 8 layers: 100, 125, 150, 175, 200, 225, 250, and 300 hPa) in a 500-km radius based on the revised TC center and defined the pressure layer with the maximum radial velocity as the outflow layer. The average geopotential height within 500 km around the revised TC center of the outflow layer is the TC outflow height. Moreover, the average temperature within 500 km around the revised TC center of the outflow layer is the TC outflow temperature.
2.5
Factors contributing to TC outflow temperature change
We calculate the TC outflow temperature change caused by the outflow height increase and troposphere warming, respectively. First, the time-averaged (1959–2021) vertical gradient of temperature between 100 and 300 hPa is calculated by using the ERA5 reanalysis data with a vertical resolution of 25 hPa. Subsequently, the contribution of the outflow height change is calculated by multiplying the increase of TC outflow height and the time-averaged vertical gradient of temperature in the corresponding layer to obtain the TC outflow temperature change caused by the outflow height change. Finally, we measure the contribution of troposphere warming by multiplying the rate of temperature (100–300 hPa) increase and time length (i.e., 63 yr, 1959–2021) to obtain the value of temperature change of the outflow layer caused by the troposphere warming.
where PI is defined as the low-level maximum wind speed of a TC (Vmax), Ts is the SST, T0 is the mean TC outflow temperature (temperature at the level of neutral buoyancy), Ck is the exchange coefficient for enthalpy, Cd is a drag coefficient; the CAPE of a saturated parcel lifted from Ts at TC eyewall pressure is denoted as CAPE∗ (saturation CAPE), and that of a parcel with environmental relative humidity, surface air temperature, and TC eyewall pressure is denoted as CAPEm (CAPE at the radius of maximum winds). Therefore, the PI can be obtained based on the SST, sea level pressure (SLP), vertical atmospheric temperature, and mixing ratio.
3.
Results
3.1
TC Outflow height
Figure 2 shows the time variation of TC outflow height in the WNP from 1959 to 2021, with a total of 47,237 TC records counted (1826 TC number). Note that TC records are not TC numbers but TC moments with a 3-h interval. For example, if a TC has a lifetime of 7 days and 56 moments appear on the ERA5 reanalysis profile, this TC is recorded 56 times. If two TCs coexist in the WNP, that moment is recorded twice. The TC outflow height rose significantly in the past decades (48.20 ± 22.18 m decade−1), and the results exceeded the 95% confidence level. The mean outflow height of the WNP TC is 14,133 m, and the annual mean outflow height has risen by 299 m from 1959 to 2021.
Fig
2.
Time evolution of TC outflow height (m) from 1959 to 2021. All calculations are done over the WNP based on the ERA5 reanalysis data.
The variation of TC outflow height shows a clear interannual oscillation and interdecadal variations, which may be related to the large-scale climate variability in the WNP (e.g., ENSO, IPO, PDO, and AMO; Camargo et al., 2007; Liu and Chan, 2008; Mei et al., 2015). Following the regressing method in Kossin et al. (2014), we derive the residuals of the annual mean TC outflow height subtracting regressed outflow height with respect to the indices (ENSO, POD, IPO, and AMO). After the effect of large-scale climate variability is removed, there is still an obvious increase trend in TC outflow height (Fig. 3), which is similar to Fig. 2.
Fig
3.
Time evolution of TC outflow height (m) from 1959 to 2021 excluding large-scale climate variability. (a) ENSO, (b) PDO, (c) IPO, (d) AMO, and (e) all. All calculations are done over the WNP based on the ERA5 reanalysis data.
Possibly due to the tropical expansion, the WNP TCs have systematically migrated poleward in the past decades (Kossin et al., 2014; Kim et al., 2015; Sun et al., 2017a). This may contribute to the changes of outflow height, as TC tropospheric height is closely related to latitude, and outflow height tends to be consistent with tropospheric height. To exclude the effect of the poleward shift on TC outflow height, we derive the residuals of the annual mean TC outflow height removing the TC latitude trend to exclude the effect of TC track poleward shift, and the result (Fig. 4) is similar to those in Figs. 2, 3, which also exhibits an increasing trend and exceeds the 95% confidence level. This indicates that the TC poleward shift does not effectively cause the rise in TC outflow height.
Fig
4.
Time evolution of TC outflow height (m) from 1959 to 2021 excluding the effect of TC track poleward shift. All calculations are done over the WNP based on the ERA5 reanalysis data.
Both observational data and numerical model results show an increase in TC intensity and strong-TC frequency in the past decades (Emanuel, 2005; Knutson et al., 2008). To investigate the impact of global warming on the outflow height of TCs with different intensities, we divide TC records into five categories by intensity (tropical depression: 10.8–17.1 m s−1; tropical storm: 17.2–24.4 m s−1; severe tropical storm: 24.5–32.6 m s−1; typhoon: 32.7–41.1 m s−1; and severe typhoon: 41.4 m s−1 or stronger) based on the TC intensity information in the IBTrACS dataset. Note that the classification of TC records here is not based on the strongest moment of the TC, but on the specific intensity of each moment throughout the TC’s life cycle. For example, a TC with a life cycle of 7 days is recorded a total of 56 times (all data in this study have a 3-h interval), of which 28 are tropical depressions, 13 are tropical storms, 7 are severe tropical storms, 5 are typhoons, and 3 are severe typhoons. Figure 5 depicts the changes of outflow height for different-intensity TCs over the WNP from 1959 to 2021. For all categories of TCs, their outflow heights all demonstrate an overall uptrend in the past decades, and these trends have all passed the 95% confidence level. Besides, the rising trend of outflow height in stronger TCs is more pronounced than that in weak TCs: the outflow height rise of tropical depression is 48.81 m decades−1, tropical storm 43.68 m decades−1, severe tropical storm 45.42 m decades−1, typhoon 62.06 m decades−1, and severe typhoon 61.09 m decades−1. Moreover, the multi-year average outflow height also rises with TC intensity: average outflow height of tropical depression in the 63 years is 14,067.9 m, tropical storm 14,093.8 m, severe tropical storm 14,135.1 m, typhoon 14,183.7 m, and severe typhoon 14,343.9 m; this is consistent with the study of Biondi et al. (2013).
Fig
5.
Time evolution of TC outflow height (m) from 1959 to 2021 for TCs of different intensity: (a) tropical depression, (b) tropical storm, (c) severe tropical storm, (d) typhoon, and (e) severe typhoon. All calculations are done over the WNP based on the ERA5 reanalysis data.
As suggested by He and Zhou (2020), the geopotential height will rise with global warming. To investigate whether the rise of outflow height is caused by TC activities or by the increase of geopotential height associated with global warming, Fig. 6 shows the geopotential height change over the WNP from 1959 to 2021. It is clear that the geopotential height rose in the past decades at the 95% confidence level, which is consistent with the results of He and Zhou (2020). The rise of geopotential height is 60–80 m in each pressure layer, while the rise of TC outflow height is about 300 m. Therefore, the rise of TC outflow height is still notable after excluding the influence of geopotential height increase.
Fig
6.
Time evolution of geopotential height from 1959 to 2021: (a) 300, (b) 250, (c) 225, (d) 200, (e) 175, (f) 150, (g) 125, (h) 100, and (i) 70 hPa. All calculations are done over the WNP based on the ERA5 reanalysis data.
3.2
Possible influences of the outflow height uptrend on outflow temperature
Outflow temperature, as an important factor associated with TC intensity variation, has been thoroughly investigated in previous studies (Emanuel et al., 2013; Ge et al., 2018). Emanuel et al. (2013) suggested that the TC outflow temperature is directly related to TC intensity and that a decrease in outflow temperature leads to an increase in TC intensity. Ge et al. (2018) indicated that the change of TC outflow temperature may be linked with TC rapid intensification in the WNP. Similar to Fig. 5, Fig. 7 shows the trend of outflow temperature in the past decades. The TC outflow temperature shows a decreasing trend from 1959 to 2021 (Fig. 7a), and it has passed the 95% confidence level. The outflow temperature of all the TCs also display a decreasing trend, but not all of them passed the 95% confidence level.
Fig
7.
Time evolution of TC outflow temperature (K) from 1959 to 2021 for different-intensity TCs. (a) Total TCs, (b) tropical depression, (c) tropical storm, (d) severe tropical storm, (e) typhoon, and (f) severe typhoon. All calculations are done over the WNP based on the ERA5 reanalysis data.
Tropospheric temperature has an important influence on the development of TC (Emanuel and Rotunno, 2011; Molinari et al., 2014). Previous studies have shown that the tropospheric temperature tends to increase under the influence of global warming (Spencer et al., 2007; Fu et al., 2011; Wu et al., 2015). We examine the tropospheric temperature change in the WNP from 1959 to 2021 by using the ERA5 reanalysis data. The temperature at 200–300 hPa, which is the typical representative pressure layer of the troposphere (Fu et al., 2011), has a clear increasing trend and exceeds the 95% confidence level (Figs. 8a–d), which is consistent with previous studies (Spencer et al., 2007; Fu et al., 2011; Wu et al., 2015). The temperature at 70–100 hPa, i.e., the top of the troposphere or higher layer, has a clear decreasing trend (Figs. 8h–i). In Bister and Emanual (2002a), the temperature above TC at 70–100 hPa showed a positive correlation with the TC outflow temperature. This decreasing trend located at 70–100 hPa can partially explain the decrease of TC outflow temperature.
Fig
8.
Time evolution of troposphere temperature from 1959 to 2021 at (a) 300, (b) 250, (c) 225, (d) 200, (e) 175, (f) 150, (g) 125, (h) 100, and (i) 70 hPa. All calculations are done over the WNP based on the ERA5 reanalysis data.
As mentioned above, we found that the TC outflow height tends to rise in the past decades over the WNP, which could contribute to the decrease of outflow temperature. Although the tropospheric temperature increases with global warming, the decrease of outflow temperature due to the increase of TC outflow height will offset this effect, which could partly explain the contradiction between the increase of tropospheric temperature and the decrease of TC outflow temperature in previous studies (Emanuel et al., 2013; Ge et al., 2018).
To further clarify the contradiction, we calculate the temperature changes caused by increasing outflow height and troposphere warming, respectively. Table 1 shows the specific influence and contribution of the tropospheric warming and the outflow height rising to TC outflow temperature. From Table 1, it is clear that the temperature decrease caused by the rise of outflow height is much greater than the increase of troposphere temperature related to global warming, which finally leads to the decrease of TC outflow temperature.
Table
1.
Contribution of TC outflow height and troposphere temperature changes to TC outflow temperature change for TCs of different intensities
According to the Carnot cycle theory of Emanuel (1986,2003), a larger difference between the temperature of the outflow height and the underlying SST near the TC is beneficial to TC development and thus to a stronger TC potential intensity. The rise of SST in the past decades has been generally recognized (Chan and Wu, 2015). In order to show the relationship between TC outflow variation and TC intensity more visually, Fig. 9 depicts the variation of the difference between underlying SST and outflow temperature in the past decades over the WNP. The difference between SST and outflow temperature exhibits an increasing trend. All TC categories pass the 95% confidence level except tropical storm and severe tropical storm. According to Downs and Kieu (2020), the increased difference indicates a more unstable atmospheric condition promoting the convection around TC, and thus favors the increase of TC intensity.
Fig
9.
Time evolution of the difference between TC underlying SST and TC outflow temperature (K) from 1959 to 2021 for TCs of different intensities. (a) Total TCs, (b) tropical depression, (c) tropical storm, (d) severe tropical storm, (e) typhoon, and (f) severe typhoon. All calculations are done over the WNP based on the ERA5 reanalysis data.
3.3
Possible influences on the TC potential intensity
As shown in Eq. (1), outflow temperature and SST are important factors for TC PI. In the above sections, we argued that the outflow height of TC rose in the past decades, which led to a lower outflow temperature of TC and a wider gap between the underlying SST and outflow temperature. These changes compromise the atmospheric stability and enhance the TC convection, leading to an increase in TC intensity. Furthermore, the rising SST induced by global warming will also lead to an increase in TC intensity (Sun et al., 2014).
To compare the influence of outflow variations and that of SST on TC PI, we calculate the annual average TC PI and the residual of TC PI by eliminating the effect of outflow height or SST over the WNP from 1959 to 2021 (Fig. 10). Note that we only calculate the PI at the initial moment (each TC is calculated only once). It is meaningless to calculate the PI at the later stage of the TC, because the ocean environment is no longer able to nurture TCs when they move to higher latitudes.
Fig
10.
Time evolution of (a) TC potential intensity PI (m s−1), (b) residual of potential intensity (PI) excluding the effect of SST warming (m s−1), and (c) residual of PI excluding the effect of TC outflow height uptrend (m s−1) from 1959 to 2021. All calculations are done at the initial moment.
Influenced by global warming, the PI of TC shows a significant increasing trend and exceeds the 95% confidence level (Fig. 10a). As shown in Fig. 2, the outflow height of TC has risen in the past decades, which leads to the change of TC outflow environment (e.g., outflow temperature decreases in Fig. 7) and favors the increase of TC intensity (Fig. 10a). The positive effect of TC outflow height on TC PI can be shown by comparing Figs. 10a, b. Upon eliminating the effect of TC outflow height uptrend, the confidence interval of TC PI decreases from 1.08 ± 0.52 to 0.74 ± 0.50 m s−1, although it still shows an increasing trend. Additionally, the confidence interval of TC PI decreases to 0.68 ± 0.47 m s−1 after eliminating the effect of SST increase (Fig. 10c). Thus, both outflow height rise and SST warming positively affect TC PI, albeit with a slightly smaller influence observed from the increase in outflow height.
On the contrary, TC strength may also affect TC outflow height. Biondi et al. (2013) found that stronger TCs tended to be associated with higher TC outflow height. However, it is difficult up to this point to furtherly investigate the physical mechanism between TC outflow height and TC intensity, which is in essence to a “chicken and egg” issue. In this study, the correlation coefficient between TC outflow height and TC intensity is 0.36, while the correlation coefficient between TC outflow height and SST is 0.65 (Fig. 11). These results suggest that changes in TC outflow height are mainly contributed by the change in SST rather than the changes in TC intensity. Therefore, in this study, there should be a positive feedback between TC outflow height and TC intensity; TC outflow height is not only a result of TC intensity but also a cause of TC intensity. Nonetheless, TC outflow height tends to act more as a cause in determining TC intensity under the context of global warming. This issue will be investigated in our next work.
Fig
11.
Time evolution of (a) TC outflow height (m) and underlying SST (℃), and (b) TC outflow height (m) and TC intensity (m s−1) from 1959 to 2021. All calculations are done over the WNP based on the ERA5 reanalysis data.
4.
Conclusions and discussion
With global warming, the troposphere temperature has been increasing while the TC outflow temperature has been decreasing. To clarify the contradiction, we make an investigation on TC outflow variations. Based on the IBTrACS dataset and ERA5 reanalysis data from 1959 to 2021, we found that the outflow height of WNP TCs has had a clear upward trend (48.20 ± 22.18 m decade−1), and this upward trend is more obvious in strong TCs (61.09 ± 40.92 m decade−1 for severe typhoon). Generally, the increase in TC outflow height corresponds to the decrease of outflow temperature, which could explain the rationality of the contradiction.
Results of the previous studies show that the decrease of TC outflow temperature leads to an increase in TC intensity and is closely related to TC rapid intensification in the WNP (Emanuel et al., 2013; Ge et al., 2018). In our study, the TC outflow temperature has experienced a significant decrease in the past decades, which is consistent with previous studies. Besides, we further found that the difference between underlying SST and outflow temperature is increasing year by year. According to Carnot theory, the difference between underlying SST and outflow temperature will directly affect the strength of TC (Emanuel, 1986, 2003). The more significant the difference between the two, the greater the TC intensity. In order to verify the relationship between TC outflow variation and TC intensity, we investigated the potential intensity (PI) variation of WNP TCs from 1959 to 2021, and the results show that in the context of global warming, influenced by TC outflow variation, the PI of TCs experienced an obvious increasing trend.
Under the influence of global warming, the increase of SST will input more energy to TC, favoring the enhancement of convection and the increase of TC intensity. These changes increase the outflow height and decrease the outflow temperature, which is further favorable to TC intensification (Emanuel et al., 2013; Downs and Kieu, 2020). Representative TC cases will be discussed in detail in our next study.
Previous studies have focused more on the changes of TC activity in the oceanic environment or the sea surface–air interaction. In contrast, there are few studies on the changes of the TC outflow environment. Our study is expected to shed new light on this topic, as we investigate the issue on TC outflow height change associated with TC intensity in the past decades. Our next step is to study the relationship between TC outflow volume and TC activity, and to extend the study area from WNP to the globe.
Fig.
3.
Time evolution of TC outflow height (m) from 1959 to 2021 excluding large-scale climate variability. (a) ENSO, (b) PDO, (c) IPO, (d) AMO, and (e) all. All calculations are done over the WNP based on the ERA5 reanalysis data.
Fig.
4.
Time evolution of TC outflow height (m) from 1959 to 2021 excluding the effect of TC track poleward shift. All calculations are done over the WNP based on the ERA5 reanalysis data.
Fig.
5.
Time evolution of TC outflow height (m) from 1959 to 2021 for TCs of different intensity: (a) tropical depression, (b) tropical storm, (c) severe tropical storm, (d) typhoon, and (e) severe typhoon. All calculations are done over the WNP based on the ERA5 reanalysis data.
Fig.
6.
Time evolution of geopotential height from 1959 to 2021: (a) 300, (b) 250, (c) 225, (d) 200, (e) 175, (f) 150, (g) 125, (h) 100, and (i) 70 hPa. All calculations are done over the WNP based on the ERA5 reanalysis data.
Fig.
7.
Time evolution of TC outflow temperature (K) from 1959 to 2021 for different-intensity TCs. (a) Total TCs, (b) tropical depression, (c) tropical storm, (d) severe tropical storm, (e) typhoon, and (f) severe typhoon. All calculations are done over the WNP based on the ERA5 reanalysis data.
Fig.
8.
Time evolution of troposphere temperature from 1959 to 2021 at (a) 300, (b) 250, (c) 225, (d) 200, (e) 175, (f) 150, (g) 125, (h) 100, and (i) 70 hPa. All calculations are done over the WNP based on the ERA5 reanalysis data.
Fig.
9.
Time evolution of the difference between TC underlying SST and TC outflow temperature (K) from 1959 to 2021 for TCs of different intensities. (a) Total TCs, (b) tropical depression, (c) tropical storm, (d) severe tropical storm, (e) typhoon, and (f) severe typhoon. All calculations are done over the WNP based on the ERA5 reanalysis data.
Fig.
10.
Time evolution of (a) TC potential intensity PI (m s−1), (b) residual of potential intensity (PI) excluding the effect of SST warming (m s−1), and (c) residual of PI excluding the effect of TC outflow height uptrend (m s−1) from 1959 to 2021. All calculations are done at the initial moment.
Fig.
11.
Time evolution of (a) TC outflow height (m) and underlying SST (℃), and (b) TC outflow height (m) and TC intensity (m s−1) from 1959 to 2021. All calculations are done over the WNP based on the ERA5 reanalysis data.
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Sun, Y., Z. H. Feng, W. Zhong, et al., 2024: Uptrend of the western North Pacific tropical cyclone outflow height during 1959–2021. J. Meteor. Res., 38(2), 339–350, doi: 10.1007/s13351-024-3097-y.
Sun, Y., Z. H. Feng, W. Zhong, et al., 2024: Uptrend of the western North Pacific tropical cyclone outflow height during 1959–2021. J. Meteor. Res., 38(2), 339–350, doi: 10.1007/s13351-024-3097-y.
Sun, Y., Z. H. Feng, W. Zhong, et al., 2024: Uptrend of the western North Pacific tropical cyclone outflow height during 1959–2021. J. Meteor. Res., 38(2), 339–350, doi: 10.1007/s13351-024-3097-y.
Citation:
Sun, Y., Z. H. Feng, W. Zhong, et al., 2024: Uptrend of the western North Pacific tropical cyclone outflow height during 1959–2021. J. Meteor. Res., 38(2), 339–350, doi: 10.1007/s13351-024-3097-y.
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Manuscript History
Received: 20 June 2023
Revised: 15 October 2023
Accepted: 06 November 2023
Available online: 07 November 2023
Final form: 15 November 2023
Typeset Proofs: 29 December 2023
Issue in Progress: 28 February 2024
Published online: 27 April 2024
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Abstract
摘要
1.
Introduction
2.
Data and methods
2.1
IBTrACS dataset
2.2
ERA5 reanalysis data
2.3
Revision of TC positions
2.4
TC outflow height and temperature
2.5
Factors contributing to TC outflow temperature change
2.6
TC potential intensity
3.
Results
3.1
TC Outflow height
3.2
Possible influences of the outflow height uptrend on outflow temperature
3.3
Possible influences on the TC potential intensity
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